1. Field of The invention
This invention relates to a novel type of ink for inkjet printing which is suitable for use in inkjet printers
2. Description of The Prior Art
Studies have been intensively made on so-called inkjet printers, wherein a liquid or fused solid ink is jetted such as from nozzles, slits or porous films, because of its small size, inexpensiveness and stillness. Especially, developments have been made on a single black color printer. In fact, there have been now sold on the market printers with which print of good quality is obtained on so-called ordinary papers such as a reporting paper, a copying paper and the like.
As for inks used in inkjet printers, the pH which is one of physical properties of the ink has influences on:
(1) corrosion and decomposition of materials for printer head; PA1 (2) settlement of constituent substances of the ink on storage over a long time, especially, settlement of dyes with inks wherein the dyes are dissolved; PA1 (3) clogging of a printer head with the ink at the tip thereof; PA1 (4) scorching on a heater when droplets are formed by application of a thermal energy; and PA1 (5) hue of printed images.
Thus, the pH should be stably controlled over a long term.
Japanese Laid-open Patent Application No. 57-209972 sets out an ink used in a method of forming ink droplets by application of a thermal energy, wherein the pH of the ink is adjusted to 4 to 9. The pH adjustors used in this application are all either strongly acidic or strongly basic in nature. Because the buffering effect in the pH range of 6 to 8 is not good, a problem is involved in the long term stability in pH of the ink. Japanese Laid-open Patent Application No. 1-149872 discloses the use, as a pH adjustor, of sodium borate, sodium hydrogenphosphate and sodium dihydrogenphosphate. However, the buffering agents are liable to react with polyvalent metal ion impurities to form insoluble salts, thereby causing clogging or uneveness of jetting. In Japanese Laid-open Patent Application Nos. 2-233780 and 2-233781, there are described combinations of phthalocyanine dyes or Direct Yellow 86, water-soluble organic solvents, and alkali metal salts of organic carboxylic acids whose acid dissociation constant at 25.degree. C. at a dissociation stage of 1 is not lower than 4.3. Using the combination, discoloration can be prevented at the time when ordinary papers and particularly, a paper having a high acidity is printed. However, a problem is still left in the long-term stability of pH.
As will be appreciated from the above, a number of techniques of controlling the pH of inks have been proposed but are not always satisfactory with respect to the requirements (1) to (5) set out hereinbefore.